Conventionally, Cr-stainless steels were produced by a conventional hot rolling-process which included casting a slab and hot-rolling the slab. This process had a problem that ridging (or roping) occurs in the cold-rolled thin sheet products due to a texture established during hot rolling. Then, trials were conducted in which the STC process was used to cast a thin cast strip for producing a thin sheet product in which no ridging occurs. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-176649 disclosed "Process of Producing Ferritic Stainless Steel Thin Strip Having No Roping". This publication, however, did not describe the reduction in toughness which occurs in Cr-stainless steels having a single phase structure and containing Nb, Ti, Al, and V in an amount of from 0.05 to 1.0 wt % in total. Therefore, there remains a problem that a cast strip of a Cr-stainless steel containing Nb, Ti, Al and V in the above-recited total amount has too low a toughness to be cold-rolled in a subsequent step.
Japanese Unexamined Patent Publication (Kokai) No. 64-4458 entitled "Rapid-Cooled Strip of Ferritic Stainless Steel Having High Toughness" disclosed that a cast strip having a high toughness can be produced by controlling its columnar crystal content to 70% or more, but did not consider the technological significance of the relationship between the toughness of and the precipitates in the cast strips of Cr-stainless steels containing NB, Ti, Al, and V.
The present inventors have been developing a technology of producing Cr-stainless steel thin sheet by using an STC process. As a result, it became apparent that cast strips have a poor toughness which causes cracking to occur during cold rolling of SUS 430 or other steel systems in which a .gamma.-phase is precipitated during cooling after solidification to room temperature and a martensite phase transformed from the .gamma.-phase remains at room temperature.
To prevent the precipitation of .gamma.-phase during cooling after solidification to room temperature, the present inventors produced a thin cast strip of a Cr-stainless steel with a controlled chemical composition having a .gamma.p value of 0% or less. The term .gamma.p is a parameter predicting the precipitate amount of .gamma.-phase based on the chemical composition. However, even when a Cr-stainless steel has a .gamma.p of 0% or less, there remains a problem that a cast strip has a poor toughness and is broken during cold rolling when it contains one or more of Nb, Ti, Al and V in an amount of 0.05 wt % or more in total.
The present inventors made a study and found that thin cast strips of Cr-stainless steels containing such elements and exhibiting a poor toughness contain fine precipitates with a size of 0.1 .mu.m or less. It is known that such fine precipitates harden the steel matrix and thus deteriorates the toughness.
A thin cast strip cast by an STC process contains fine precipitates of 0.1 .mu.m or less, probably because its speed of cooling after solidification to room temperature is much higher than that of a slab cast by the conventional process, so that those precipitates, which precipitate and can grow to several .mu. m during cooling of a slab by the conventional process, do not actually have sufficient time to precipitate and grow but precipitate in a fine form instead in a thin cast strip cast by an STC process.
Thus, to improve the toughness of a thin cast strip of a Cr-stainless steel containing one or more of Nb, Ti, Al and V in an amount of 0.05 wt % or more in total, it is required that precipitates be grown to 0.1 .mu.m or greater.
This problem occurrs in Cr-stainless steels containing Nb, Ti, Al and/or V in an amount of 0.05 wt % or more, irrespective of the structure of a cast strip such as the content of columnar crystals.
It also became apparent that the conventional hot-rolling process has no problem concerning the toughness of hot-rolled and annealed sheets of the subject steels of the present invention and that the problem is specific to STC processes.